Rf sputtering apparatus including a wave reflector positioned behind the target



Dec. 9, 1969 M. sAPoFF ET AL. 3,483,114

RF SPUTTERING APPARATUS INCLUDING A WAVE REFLECTOR POSITIONED BEHIND THE TARGET Filed May 1, 1967 1Lelllllml ff w United States Patent O RF SPUTTERING APPARATUS INCLUDING A WAVE REFLECTOR POSITIONED BEHIND THE TARGET Meyer Sapoff, West Orange, and John G. Froemel, Verona, NJ., assignors to Victory Engineering Corporation, Springfield, NJ., a corporation of Delaware Filed May 1, 1967, Ser. No. 634,928 Int. Cl. C23c 15/00 U.S. Cl. 204-298 8 Claims ABSTRACT OF THE DISCLOSURE SUMMARY OF THE INVENTION This invention relates to a sputtering device having an anode, a cathode, a target, and a substrate on which sputtered molecules are to be deposited. The cathode comprises a base plate and a thermionic emitter. The anode is a conductive plate coupled to the cathode in series with a direct current source. A target is mounted on one side of the discharge space between the anode and cathode and comprises a first conductive layer on which the target material is mounted. The first conductive layer is connected to a radio frequency oscillator by means of a concentric cable. A wave reflector is mounted behind the target and is parallel thereto. The reflector is conductive but is not connected to any part of the circuit. The substrate is mounted directly on the other side of the discharge space between the anode and cathode and mounted in a conductive sheet which is at ground potential.

It has been found that materials sputtered in a rarefied atmosphere having a pressure which may be varied from one to ten microns of mercury produce evenly spaced molecules on the substrate surface and thereby produce a molecular film which is not only constant in thickness but is easily reproduced. The present invention is adapted to produce films having a high coeicient of temperature resistivity, either positive or negative. These components are generally called thermistors and have important use in many fields including communication circuits, amplifiers, and measuring circuits. Ordinary sputtering techniques often consume considerable time for the deposition of a film of adequate thickness and for this reason it is important to have means for increasing the rate of deposition of the target material. The present invention accomplishes this problem by the use of high frequency voltages applied to the target and by the use of a conductive reflector placed behind the target. This array is similar to a radio antenna and produces an intense electric field in front of the target.

BRIEF DESCRIPTION OF THE FIGURE The figure is a cross sectional view showing an evacuated envelope with the anode, cathode, target, and substrate all mounted within the envelopes space.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the figure, the evacuated envelope 10 is in the form of a bell jar which is mounted on a circular gasket 11 supported by a metal base 12. A secondary base 13 is coupled to base 12 and is also used for supporting components within the envelope. At one side of the base 12 a thermal emissive means 14 is mounted. This means includes lead-in conductors 15 connected to an electron emissive filament 16. A source of potential 17 is connected to lead-in conductors 15 for heating ythe filament. The filament is supported within a conduit 1.8 connected to a cathode base 20.

At the center of base 13 a large conduit 21 is positioned for connection to a vacuum system which pumps the air out of the bell jar, aids in flushing the bell jars space with gases which clean the components inside the jar, and then helps maintain a required reduced pressure of an inert gas within the envelope. The inert gas may be introduced into the envelope by an auxiliary conduit 22 under the control of a valve 23.

An anode 24 is supported on a horizontal rod 25 and an insulated vertical rod 26. The lower end of rod 26 is brought out through an insulated sleeve in base 13 and is connected to a direct current source of potential 27. The other terminal of the source 27 is connected to the base 13 and to the cathode 20. The source 27 produces an electric discharge within the envelope 10 between the anode and cathode, the electrons moving upwardly from the cathode to the anode and thereby producing a large number of positive gas ions which travel to the nearest negative electrode, some going to the cathode 20 and others striking the target 35.

The target assembly 28 includes an electrically conductive layer 30 which may be a piece of foil attached to a quartz or glass plate 31. The plate 31 is cemented to two supports 32 and 33 which are held by a vertical rod 34 secured to the cathode. Supports 32, 33 are non-conductive and play no part in forming or maintaining the electrical discharge. On the front face of the foil 30 a target 35 is mounted, this material being the mixture of metal oxides and other desired components which are to be sputtered onto a substrate to produce the thermistor film. The target mixture is generally conductive but in some cases the resistivity is high and the target may be classed as a semiconductor.

A substrate 37 is mounted on a conductive sheet 38 opposite the target 28 and connected by means of horizontal rods -40 and a vertical support rod 41 to the cathode 20. The supporting means are conductive which means that the conductive sheet 38 is maintained at the cathode or ground potential.

If it is desired to deposit the thermistor material on the substrate in a definite pattern, a mask `42 may be positioned in front of the substrate 37. The use of a mask for this purpose is old in the art and the details of its mounting and operation need not be described here in detail.

It is important to provide a high intensity field between the target and the mask so that sufficient ionization is produced at this point. The ionization produces positively charged gas ions and these are drawn to the target whenever the radio frequency wave from oscillator 45 goes negative. This'createsy an electric field in front of the target which attracts the positive ions and causes them to bombard the target with considerable energy. The result of this bombardment is to tear out target molecules. These are then deposited on the substrate 37. The application of ratio frequency voltages to the target is accomplished vby a concentric line 46 having an outer conductor 47 and an inner Iconductor 48. The outer conductor is grounded and the inner conductor is connected between the output of the oscillator 45 and the conductive layer 30 which is positioned under the target 35. The oscillator produces a frequency with-in the range of l0() to 500 megacycles per second. The layer 30 can be compared to an antenna which produces an electric field on both sides of its plane 3 surface. It is obvious that the electric `field produced on the left of the layer 30, as ls hownuinthe ligure, accomplishes no useful purpose. For this reason a wave reflector 50 is mounted in parallel alignment with the layer 30 and reflects the waves back so that the ield intensity between the target and the substrate is materially increased. This structure results in a deposition rate, which is considerably in excess of the rate which would be obtained if the 'wave reflector were not used. The lreflector 50 is not electrically connected to any other part of the apparatus but is left oating. Adjustable means are provided for varying the distance between the reflector 50 and the conductive layer 30 so that a maximum deposition rate may be obtained.v The optimum distance between the layer 30 and the reflector 50 depends upon the gas used in the envelope 10, the thickness of the supporting insulator 31, and the frequency of the RF.

A winding 51 is placed around the envelope and connected to a source of direct current :power 52 which may be a battery. The winding 51 creates a magnetic eld ,whose axis coincides with the axis of the bell jarand constrains the electron emission from the cathode to produce an electron stream which is not only intensified but is directed to the space between the target 35 and the substrate 37. This is the intensied stream of electrons that operates to ionize the inert gas molecules within the envelope to create intense ionization between the target and substrate.

The'operation of this device has been described in some detail in the above paragraphs. In general, after the substrate 37, the mask 42, and the target 35 have been positioned, the bell jar is placed on its gasket 11, and conduit 21 is connected to a vacuum system which pumps most of the air out of the container. After a program of flushing and repumping in order to clean all components on the inside of the envelope, source 17 is connected to terminals in order to heat the emissive lament 16. Next, source 27 is connected between the anode and cathode to create a direct current discharge between these electrodes and to ionize the gas molecules. Power source 52 may next be connected to winding 51 to compress the electron iiow from the cathode and to intensify the production of positive ions between the target and substrate. The lradio frequency oscillator 45 is then connected to the concentric line 46 and the sputtering action starts.

Many types of targets can be used in order to deposit their molecules onto the substrate. An example of a composition from which ceramic thermistors can be made is as follows:

Percent by weight r After a suicient thickness of this lmaterial has been deposited on the substrate, all the gas is eliminated from the envelope, all the electric sources of power are disconnected, and then air is led into the envelope and the bell jar maybe removed. The substrate is now removed from its holder, and may be red in a furnace to cure it and then electrodes are added at desired points to provide electrical contacts for connection in an electric circuit.

It has been found that the operation of the apparatus described above with a direct current potentialconnected between the layer 30 under the target and the anodesheet 38 results in a sheath of electrons which form above the target. This sheath is generally termed a space charge and if it is not eliminated, the molecules sputtered from the target will not deposit evenly o n the substrate. The radio plV frequency oscillator eliminates this space charge'because,

during the positive halves of the radio frequency wave,

the electrons are swept away from the substrate surface and then, during the positive half cycles, the sputtered material can be applied.

Having thus fully described the invention, what is claimed as new and-desired to lbe secured by Letters Patent of the'United States is:

1. A sputtering device for depositing a desired material onto a non-conductive substrate in a thin film comprising; a sealed envelope, means for providing a reduced pressure in said envelope, a cathode mounted'in the envelope and having a supply of negative electrons associated therewith, an anode mounted in the envelope opposite to the cathode, a source of direct current power connected between the anode and the cathode for creating an electric discharge therebetween and thereby dening a discharge space, a target composed of the material to be sputtered positioned at one side of the discharge space and secured to a conductive layer, a substrate mounted on the other side of the discharge space opposite to the target for receiving the sputtered material from the target, said substrate mounted on a conductive sheet which is connected to the cathode, said conductive layer under the target connected to an oscillator for the application 'of radio frequency power, a coil of wire mounted exterior of the envelope and connected to a source of direct current power for creating a magnetic ux to concentrate the electron beam between the anode and cathode, and an electrically isolated conductive wave reflector mounted behind the target in parallel alignment therewith for reecting radiated waves transmitted by the conductive layer and for increasing thealternating electric field between the target and the substrate. l

2. A sputtering device as claimed in claim 1 wherein said envelope is connected to a vacuum pumping system and to a supply of gases for creating a desired pressure for the sputtering operation and for ushing the envelope space to clean the components therein.

3. A sputtering device as claimed in claim 1 wherein a mask is positioned between the target and the substrate for selective deposition of the target material.

4. A sputtering device as claimed in claim 1 wherein said cathode is formed with a cavity in which is positioned an electron emissive iilament.

5. A sputtering device as claimed in claim 1 wherein said oscillator produces a frequency within the range 0f to 500 megacycles per second.

6. A sputtering device as claimed in claim 1 wherein the conductive layer under the target is connected to the oscillator by means of a concentric line.

7. A sputtering device as claimed in claim 1 wherein said wave retlector is adjustable in spaced relation to the target.

8. A sputtering device as claimed in claim 1 wherein said envelope includes a bell jar having a rim detachably mounted on an annular base for affording easy access to the components within the envelope.

References Cited UNITED STATES PATENTS 3,287,243 1l/l966 Ligenza 204-298 ROBERT K. MIHALEK, Primary Examiner U.S. Cl. X.R. ZOLL-192 

